The present invention relates to fluorine-containing polymer powder utilizable in powder coating etc. and a process for producing the same.
Fluorine-containing polymer powder can be used as a powder paint. In order to obtain a processed and coated film having a smooth surface from a powder paint, particles of the powder paint should be high in apparent density, nearly spherical in particle shape and excellent in fluidity. In a method where a powder paint is obtained from fluorine-containing resin by a mechanical comminution method, it was considered that pellets obtained in a melting extruder are easily made fibrous, and it is difficult to obtain an excellent powder paint even by freeze-comminution. A method of obtaining an emulsification polymerization-type dispersion by a spray dry method requires enormous facilities and high costs, and its productivity is low.
In a process for producing transparent tetrafluoroethylene copolymer powder disclosed in Japanese Patent Kokoku Publication No. 11296/1978, the powder after classification cannot be recycled as powder for powder paint, so its productivity is not good.
For improving apparent density and fluidity by heating, there is a method of obtaining an aqueous dispersion by spray-drying as described in Japanese Patent Kokoku Publication No. 11296/1978, but the facilities used therein are enormous and costs for the facilities are also high. Further, a method of separating and crushing powder which was heat-fused by heat treatment for a long time at a temperature ranging from the fusion-initiating temperature to the melting point of a copolymer, as described in Japanese Laid-Open Patent Publication No. 202329/1992, makes use of heat treatment by a batch furnace on a metal vessel (tray) or an indirect heating-type rotary kiln etc., but due to the heat-fusion of the powder, metal pollution occurs at a position where the powder is in contact with the metal, so the product is not preferable for uses in semiconductor industry.
Powder after polymerization and drying of fluorine-containing polymer powder often forms aggregated fine powder or ball-shaped granulated powder having a diameter of more than 1 mm by suspension solvent, so uniform powder particles cannot be obtained in many cases, and it is known that an extruded strand obtained by a kneader is cut into pellets which are then pulverized to give powder for powder coating. However, the fine powder obtained by comminuting the pellets is fibrous particles due to strong stress at the time of comminution, so the powder suffers from the problems of low apparent density and poor powder fluidity.
The object of the present invention is to provide fluorine-containing polymer powder which can be used as a coating powder that is excellent in productivity, is excellent in powder fluidity, and does not generate pollution upon heat-fusion, as well as a process for producing the same.
The present invention provides a process for producing fluorine-containing polymer powder which comprises densifying fluorine-containing polymer raw powder with rolls under conditions for permitting its specific gravity to achieve at least 90% of the true specific gravity, then comminuting the densified material, removing fine particles in the range of 3 to 40% by weight of the whole particle size distribution of the comminuted material by air classification, and further removing coarse particles in the range of 1 to 20% by weight of the whole particle size distribution of the comminuted material by classification.
The present invention also provides a process for producing fluorine-containing polymer powder which comprises densifying fluorine-containing polymer raw powder with rolls under conditions for permitting its specific gravity to achieve at least 90% of the true specific gravity, then comminuting the densified material, removing fine particles in the range of 3 to 40% by weight of the whole particle size distribution of the comminuted material by air classification, further removing coarse particles in the range of 1 to 20% by weight of the whole particle size distribution of the comminuted material by classification, and conducting heat-treatment at a temperature of not less than the fusion-initiating temperature of the fluorine-containing polymer powder.